It is desirable to process audio signals delivered by an electronic organ by frequency modulating and amplitude modulating the signals concurrently at a sub-audio frequency, thereby simulating the effect of a Leslie rotating accoustic radiator but without requiring mechanical devices. It is conventional to employ a transistor circuit as a modulated amplifier, and such circuits exist which are capable of responding to a large range of modulation signal amplitudes and delivering a wide range of signal output amplitudes without distortion or passing through of the modulating signals.
In accordance with one embodiment of the present invention, a first transistor is provided with audio signal, which may be derived from an electronic organ, into its emitter through a resistance large relative to base-emitter resistance. Dc and sub-audio modulating signals are applied to the base of the transistor, and thereby both modulate the amplitude of the audio signal at the collector of the transistor, but do not affect each other, so that each may be independently selected in respect to amplitude and will separately and independently modulate the audio signal. The modulating signals are cancelled by means of a second transistor amplifier, but the modulated signal is not cancelled.